The terms positioning and navigation in a wireless communication system are connected to technology with the purpose of determining the geographic position of an object, equipment or a person carrying the equipment. The position is given with respect to a specified coordinate system. A common approach is to use positioning by means of measuring the arrival times of radio waves from a number of not co-located transmitters, to the positioned object. This is the basic principle of many positioning approaches, e.g. the so-called Observed Time Difference Of Arrival—Idle Period Down Link (OTDOA-IPDL) positioning method that has been standardised by 3GPP. However, the proposed technology is also applicable and useful for multi-carrier CDMA and other CDMA systems.
The positioning of a mobile unit can be performed for many purposes. The most obvious purpose is perhaps the desire to have a possibility to determine the position of a mobile unit signaling an emergency message. Other purposes could be more business directed, such as e.g. providing site specific advertising information over a mobile telephone. The need for positioning accuracy is already today well investigated. There are e.g. stringent coverage and accuracy requirements on emergency positioning methods in North America.
In particular, user equipment (UE) based methods (to which OTDOA-IPDL belongs) require a 50 m absolute accuracy for 67% of all UE's within the network. It is therefore clear for anyone skilled in the art that very small additional errors can be tolerated on top of the raw measurement accuracy (of the time of arrival measurement in the UE). This measurement accuracy, which in practice is of about 10% of a chip (10 m), is further amplified with geometric effects, e.g. so called VDOP (Vertical Dilution of Precision) and HDOP (Horizontal Dilution of Precision). These factors are typically of the order of 1.5–3, meaning that the raw measurement accuracy is already relatively close to the 50 m limit. Multipath and none-line of sight (LOS) propagation constitute other major error sources.
Very briefly, the OTDOA method relies on measurements on so called CPICH (Common PIlot CHannel) radio signals from multiple sites. The measurement is performed by means of correlation with the known CPICH signals of the sites (and cells) measured upon. Assuming for the moment that measurements of CPICH timing are successful for a number of sites, the following relations between detection times, transmission times and the distances between the UE and the sites follow:
                                          t            1                    +                      t            clockbias                          =                              T            1                    +                                                                                    r                  1                                -                                  r                  UE                                                                    /            c                                          ⋮                                                t            n                    +                      t            clockbias                          =                              T            n                    +                                                                                    r                  n                                -                                  r                  UE                                                                    /                          c              .                                          
Here t1, i=1, . . . , n denotes the measured time of arrivals in the UE, Ti, i=1, . . . , n denotes the transmission times from the node B's and c is the speed of light. The clock bias (error) originate from the fact that network and UE times may not be perfectly synchronized. The boldface quantities are the (vector) locations of the sites and the UE. In order to remove the clock bias, in the OTDOA method, time of arrival differences with respect to the own site can be formed according to:
                              OTD          21                =                                            t              2                        -                          t              1                                =                                    T              2                        -                          T              1                        +                                                                                                r                    2                                    -                                      r                    UE                                                                              /              c                        -                                                                                                r                    1                                    -                                      r                    UE                                                                              /              c                                                      ⋮                                    OTD          n1                =                                            t              n                        -                          t              1                                =                                    T              n                        -                          T              1                        +                                                                                                r                    n                                    -                                      r                    UE                                                                              /              c                        -                                                                                                r                    1                                    -                                      r                    UE                                                                              /                              c                .                                                        
In these n−1 equations, the left hand sides are basically known (with some additional measurement error). The time of transmission differences (denoted the real time differences, the RTD's) can be measured by one of two methods. In the method preferred by Ericsson, a GPS receiver in each RBS (Radio Base Station) measures the absolute GPS time with a high accuracy. This is used in order to time stamp the time of transmission from the RBS, and hence the real time differences (with some measurement errors) are known also. A second method uses a WCDMA receiver at a pre-determined location that measures the RTD's indirectly, by measuring OTD's and then calculating the RTD's. Furthermore, the locations of the sites, r1, i=1, . . . , n, can be surveyed to within a few meters and so they are accurately known as well. What remains unknown is the UE location. In case a two-dimensional positioning is requested, i.e. a lateral unknown position is sought, the UE location to be computed is:rUE=(xUE yUE)T.It then follows that at least two time of arrival differences are needed in order to find a UE position. This, in turn, means that at least three sites need to be detected for UE positioning. However, in cases where only the absolute minimum number of sites are detected, there may be multiple (false) solutions. In such cases, at least one more site has to be added, i.e. totally four sites.
As indicated indirectly above, it would be beneficial if the position determination could be improved over the measurement accuracy. In theory, the accuracy of the positioning can be improved if more measurements are collected and a maximum likelihood solution is introduced. However, also this procedure involves more than the minimum number of sites.
In practice, the so-called near-far problem makes it troublesome for a UE in a CDMA system to detect neighbour cells in large portions of the own cell. This is because the users of CDMA systems all share the same frequency and hence the own cell transmission drowns the weaker signals from neighbour sites. By assuming a certain geometrical pattern of the wireless communication system sites, it is possible to calculate the percentage of the area of the own cell, in which a certain signal to interference ratio or Ec/IO for detecting neighbouring cells exist. Present requirements of 3GPP call for a UE measurement capability down to approximately −20 dB Ec/IO.
Under certain realistic conditions, see Ericsson, “IPDL simulation for time mask evaluation”, R4-020118 TSG-RAN WG4, meeting #21, Sophia Antipolis, France, Jan. 28–Feb. 1, 2002, it can be shown that one neighbour site can be detected in 48% of the own cell, two neighbour sites in 19% of the own cell and three neighbour sites in 4% of the own cell. Thus lateral positioning is possible in 19% of the own cell (assuming no false solutions) and improved positioning with support from one extra cell is possible in only 4% of the own cell. This is obviously not acceptable and something has to be done to reduce the interference from the own cell.
One way to improve these conditions is to use the IPDL method. This approach solves the problem by turning off/attenuating the power from the own cell during very short periods, typically one slot, which is equal to 667 microseconds or 2560 chips of the WCDMA signal. This reduces the interference significantly. When using IPDL, one neighbour site can be detected in 96% of the own cell, 2 neighbour sites can be heard in 75% of the own cell while 3 neighbour sites can be heard in 45% of the own cell. This is a significant improvement, but not totally satisfactory. Since some improvement of the positioning accuracy is required in many cases, such improved positioning can only be obtained in at most 45% of the own cell.
In mountain areas or in extreme urban areas, where the terrain has a significant vertical extension, a positioning determination based on lateral coordinates may often suffer from additional errors due to that the UE is movable not only laterally, but also vertically. Also additional horizontal positioning errors result in these cases.